The field of the invention relates to fault detection for mechanical-electrical position sensors.
In motor vehicle applications in particular, throttle control systems utilize position sensors for providing an electrical signal related to the position, or throttle angle, of the engine throttle plate. Examples of such control systems include, speed control systems, wheelslip control systems, and drive-by-wire control systems. In a typical drive-by-wire control systems, an error signal is derived by comparing a desired throttle angle signal to an actual throttle angle signal from the throttle position sensor. A servo motor adjust the engine throttle in response to the error signal. In some drive-by-wire systems, the desired throttle angle signal is provided by another position sensor coupled to an actuator such as the vehicle accelerator pedal. If either position sensor is faulty, undesired engine operation may result.
The position sensors are defined by a potentiometer having a main resistor coupled across a source of electrical power and a wiper arm mechanically coupled to the control device (throttle plate or accelerator pedal) and electrically coupled to the main resistor. Thus, a conventional resistive voltage divider network is formed wherein the voltage at the wiper arm is proportional to position of the control device.
A conventional fault detection scheme for a potentiometer sensor is disclosed in Japanese patent reference No. 59-58124 issued to Mitsuhiko. The wiper arm of the potentiometer is coupled to an operational amplifier configured as a voltage comparator. A fault indication is provided when the sensor output either exceeds a voltage associated with the throttle open position or falls below a voltage associated with the throttle closed position. In response to the fault indication, a fault indicator lamp is actuated.
The inventor herein has recognized numerous disadvantages with the prior fault detection approaches. Since output voltage at the wiper arm is compared, and this voltage varies with position of the control device, these approaches are limited to detecting faults which occur beyond the voltage range associated with the operating range of the control device. Stated another way, prior approaches are limited to detecting catastrophic failures such as shorts or opens in the main resistor. For example, if the main resistor is partially shorted to ground, prior approaches will only detect a fault when the throttle moves to near a throttle closed position. During normal vehicle driving, a fault indication may not be provided even though the throttle control system is behaving erratically. Similarly, if the main resistor is partially shorted to the voltage source, prior approaches will only detect a fault when the throttle moves near a throttle open position. Further, prior approaches may not detect partial impairment of the main resistor which may result in erratic control system behavior.